Method of coating silicon steel in conjunction with box annealing thereof preparatory to die punching



United States Patent No Drawing. Filed Dec. 10, 1962, Ser. No. 243,582

2 Claims. (Cl. 204-145) This invention relates to the separating mediumcoatings employed to prevent adhesion of adjacent wraps of steel stripduring box annealing of the material in coil form, and is particularlyconcerned with removable coatings which do not impair die life inpunching operations.

Typical of products requiring the application of a separating mediumcoating during production, and which will be subjected to die-formingoperations, are grainoriented silicon steel electrical sheets, which arefabricated into laminations of suitable shape to form the magnetic coresof electrical equipment, such as transformers and generators. In theproduction of such sheets, the steel is conventionally subjected to hotand cold rolling operations, which are followed by box annealing in coilform at a temperature of about 1800 to 2200 F., in hydrogen. A magnesiacoating is usually applied to prevent adhesion of the adjacent wraps ofthe coiled material. This coating has been found to be the cause ofpremature Wear of dies employed in fabrication of the material for itsintended purpose. The magnesia coating, during the high temperature boxanneal, forms a glassy-like iron-magnesium-silicate coating whichimpairs the punching quality of the sheet material, and is difiicult toremove.

It is, therefore, an object of our invention to provide a coating thatachieves the separating-medium protection required during box annealing,but may be completely removed thereafter.

Also, it is an object of our invention to prevent the formation of theaforementioned glass-like magnesium.- silicate coating.

It is a further object of our invention to provide a pickling procedurefor the substantially complete removal of said coating, after it hasserved its purpose, and prior to die-forming operations.

Coating mixtures containing magnesium oxide (MgO) and sodium chloride(NaCl), in which the latter is present in the proportion of about one(1) part in six (6) to one (1) part in twenty-five (25) of the weight ofMgO, have been found highly satisfactory when applied in coatingweights, of residual solids, of about 0.010 to 0.030 ounce per squarefoot of strip material.

Such coatings may be obtained from solutions in which the MgO rangesfrom about to 15 ounces per gallon of water, preferably from about 7 to11 ounces. At MgO contents substantially outside of these ranges it ismore difficult, by usual procedures, to obtain resulting coatings ofdesired weights. At MgO contents substantially above about ounces pergallon, the solutions tend to become diflicult to handle, by virtue oftheir high viscosity.

It has been found that grain-oriented silicon steel, conventionallyprocessed by rolling and annealing but employing the foregoingseparating medium, may be cleaned of the medium by pickling in aqueousnitric-sulfuric acid mixtures. Satisfactory aqueous solution strengthsrange from about 10 to 20 percent by volume of concentrated nitric acid(sp. gr, 1.42 at 68 F.) with concentrated sulfuric acid (sp. gr. 1.84 at68 F.) being at about 50% of the nitric acid, by volume.

'ice

This solution is employed at about to 110 F., preferably about 85 to F.,under which conditions coatings of this invention are completelystripped upon immersion for 30 to 90 seconds. Complete removal wasobtained electrochemically in the same solution by pickling for 10seconds anodic and 5 seconds cathodic at a current density within therange of 50 to 160 amperes per square foot, preferably about amperes persquare foot.

Some illustrative test results follow, wherein grainoriented electricalsheets containing about 3.0 percent silicon received the followingcoatings:

Solution: ounces per Coating weight gallon of distilled water applied:ounces per square foot of residual solids MgO NaCl The steel was thenbox annealed at a temperature of 2050 F. in a hydrogen atmosphere. Theresulting coatings were examined by means of X-ray analysis and werefound not to contain glass-like magnesium-silicate compound.

Samples of the aforementioned silicon steel, coated and annealed asoutlined above, were submitted to the following pickling procedures:

(1) Samples coated with each mixture were immersed in a solution of 20percent (vol.) HNO and 10percent (vol.) H 50 for 30 seconds at atemperature of 90 F.

(2) Identical samples were immersed in the same solution at the sametemperature as (1) but were electrochemically pickled for 10 secondsanodic, 5 seconds cathodic at a current density of 100 amps. per squarefoot. In all of the above pickling tests the coatings were completelyremoved.

In punching tests under conditions where conventionally processedelectrical sheets caused objectionable die burring in 1,000 to 3,000punchings, sheets processed in accordance with the invention permittedapproximately 80,000 punchings before an objectionable burr on the dieresulted.

While specific examples and test results have been presented, these areto be understood as illustrative only, for purposes of readycomprehension and practice of the invention and not as limitationsthereof or excluding such variations and modifications as would occur toone familiar with the art to which the invention appertains.

We claim: 1. In conjunction with box annealing of silicon steel strippreparatory to die punching, the steps of (a) coating said steel stripwith a composition comprising essentially MgO and NaCl in which the NaClis in the proportion of about one (1) part in six (6) to one (1) part intwenty-five (25) of the weight of MgO;

(b) box annealing said coated strip; and

(c) subsequently removing the coating resulting from said treatment bypickling in a composition comprising nitric and sulfuric acids.

2. The process of claim 1 in which the strip or the like is madesuccessively anodic and cathodic during pickling.

(References on following page) 3 4 References Cited by the Examiner2,772,180 11/1956 Neel et 211. 106-286 2,863,792 12/1958 Todd .Q 1343 XUNITED STATES PATENTS 2,939,826 6/ 1960 Gulick 204145 11/ 1930 Moms3,039,902 6/1962 Miller et a1. 14827 6/1938 MeCulloch 148-22 X 5 4/ 1940Wilson 204145 JOHN H. MACK, Primary Examiner. 8/1954 Emmett et a1.204145 R. K. MIHALEK, Assistant Examiner.

1. IN CONJUNCTION WITH BOX ANNEALING OF SILICON STEEL STRIP PREPARATORYTO DIE PUNCHING, THE STEPS OF (A) COATING SAID STEEL STRIP WITH ACOMPOSITION COMPRISING ESSENTIALLY MGO AND NACI IN WHICH THE NACI IS INTHE PROPORTION OF ABOUT ON (1) PART IN SIX (6) TO ONE (1) PART INTWENTY-FIVE (25) OF THE WEIGHT OF MGO; (B) BOX ANNEALING SAID COATEDRESULTING FROM (C) SUBSEQUENTLY REMOVING THE COATING RESULTING FROM SAIDTREATMENT BY PICKLING IN A COMPOSITION COMPRISING NITRIC AND SULFURICACIDS.
 2. THE PROCESS OF CLAIM 1 IN WHICH THE STRIP OR THE LIKE IN MADESUCCESSIVELY ANODIC AND CATHOLIDE DURING PICKLING.